During cardiac surgery, circulation of blood through a patient's body may be maintained by connecting the patient to an extracorporeal system, such as a heart-lung machine. The heart-lung machine adds oxygen to and removes carbon dioxide from the blood, heats or cools the blood, and provides impetus to the blood to cause the blood to circulate through the patient's vascular system.
Connecting a patient to an extracorporeal system is typically done by inserting a cannula into the patient's venous system near or in the heart to remove blood from the patient and direct it to the extracorporeal circuit. After the blood has passed through the extracorporeal circuit, the blood is infused into the patient's arterial system near the heart.
The venous cannula that is inserted into the heart to siphon blood away for entry into the heart-lung machine is typically inserted into the right atrium and/or vena cava. The venous cannula may be a single stage device having one set of input apertures or a multiple-stage device used to simultaneously drain the right atrium and superior vena cava through an atrial basket while the inferior vena cava is drained through another set of apertures at the distal tip of the cannula. Oxygenated blood is returned to the heart from the heart-lung machine using an arterial cannula positioned in the aorta.
Regardless of the type of surgical procedure in which a cannula is being used, the outside diameter of the cannula should be as small as possible with the largest possible inside diameter in order to maximize the flow of blood to and from the patient. The wall thickness of the cannula is therefore desired to be as thin as possible to maximize flow volume. However, a cannula must have the stiffness required to be inserted into the patient. The cannula may have to be flexed or bent as it is inserted into the proper location in a patient's body. The cannula must also be able to withstand negative pressure applied to the lumen without kinking/collapsing. The negative pressure occurs when blood is drawn from the patient from a gravity siphon, a vacuum assist, kinetic suction or the like, applied to the cannula to pull the blood into the extra-corporeal circuit.
Whether the cannula is being used to drain or insert fluids, it is desirable to maintain proper fluid flow through the cannula at all times. Accordingly, it is advantageous to minimize the wall thickness while preventing kinking of the tube. Kinking of a cannula occurs when a tube is flexed and results in the sides of the tube touching each other and folding in half, thus blocking or minimizing fluid flow through the interior lumen. Cannula materials, design, and aperture placement are chosen to minimize such kinking. Cannula may be made of different materials having a more resilient tip and a stiffer proximal section to accommodate handling. Additionally, external reinforcement has been used, such as a reinforcing spring integrated into the walls of the cannula, to prevent collapse of the lumen when the cannula is flexed.
Another challenge of cannula design is the minimization of buckling of the apertures in the walls of the cannula. Typically apertures are punched or drilled into the walls of a cannula to permit flow into or out of the lumen. Many apertures may be used in order to improve the drainage or perfusion characteristics of the cannula. When the cannula is flexed during placement of the cannula into the body, such as when inserting a cannula into the inferior vena cava or right atrium, the apertures may buckle. Buckling is the phenomenon of the sides of individual cannula apertures puckering outward when the cannula body is flexed. It is preferable to maintain a smooth outer surface on the cannula to minimize trauma to the tissue when the cannula is moved.
Referring to FIGS. 1 and 2, when a cannula 1 is flexed, aperture buckling can occur. The sides 3, 4 of individual apertures 5 on the concave side 8 are necessarily pushed toward one another as the cannula 1 is bent. As the sides 3, 4 close toward one another, the apertures 5 may buckle outward at other sides 6, 7.
The buckling phenomenon is undesirable because the portion of the aperture that buckles outward creates a scoop that extends outward from the cannula wall and may damage the sides of a vessel wall in the patient. For example, a venous cannula must be flexed as it is guided into the right atrium and the vena cava when performing a cardiopulmonary bypass procedure. It is desirable to minimize tissue damage to the internal vessel walls due to the puckering of apertures in the cannula as the cannula is placed into position. In addition, the doctor may adjust the cannula during a procedure to replace it into the desired location after manipulating the surrounding tissue to accommodate the procedure. For example, the doctor may lift and move the heart to allow visual access to the back of the organ for sewing. The cannula may slide out of position in this procedure and need to be adjusted. A buckled aperture interrupts the smooth outer surface of the cannula and may cause more trauma to the surrounding tissue if it is rubbed against a sidewall.
Conventional cannula designs attempt to minimize kinking of the cannula and buckling of flow apertures through the use of different materials such as the use of a hard plastic insert in the cannula that contains the flow apertures and a helical reinforcing spring to increase kink resistance. However, it is desirable to enhance cannula flexibility while also minimizing kinking of the cannula and buckling of the cannula apertures. A reinforcing wire is used to prevent kinking of the cannula in some designs but creates dimension limitations. It either reduces the flow for a given outside diameter cannula or increases the outside diameter required to achieve a desired flow. Therefore, it may be desirable to omit the reinforcing wire at the distal end of the cannula where the flow apertures reside. Simultaneously, it is desirable to maintain similar flow characteristics through the flow apertures while minimizing the chances of the apertures buckling when the cannula is bent or flexed.
There is a need for a cannula design that is flexible yet resistant to kinking. Further, there is a need for a cannula having flow apertures that resist buckling when the cannula is flexed. It would be desirable for a cannula design or method of cannula manufacture to provide one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.